Nitrogen is an essential nutrient for supporting the growth and development of plants, including gasses. Most plants grown to produce food, either for human or animal consumption, are given some form of nitrogen fertilizer. Fertilization of lawns also consumes large amounts of nitrogen fertilizers.
Urea continues to be the most widely used source of nitrogen fertilizer. While most of the urea is used in a granular forms liquid fertilizers using urea in some form continue to occupy an important segment of the fertilizer market. Probably the most common of the urea-based liquid fertilizers are aqueous urea solutions and an aqueous solution of urea and ammonium nitrate, identified as UAN solutions (Ammonium nitrate (AN) solutions without added urea also are used to some extent as a nitrogen fertilizer). The most concentrated of these aqueous solutions contains about 32% by weight nitrogen and is made from about 34 to 35% urea, 46 to 45% ammonium nitrate and the balance water. This concentrated fertilizer solution has a salt-out temperature of about 0 to −2° C., which limits the locations where it can be safely used without added complexity in transportation and storage. The salt-out temperature can be depressed further by increasing the water content and thus sacrificing the total nitrogen concentration of the aqueous fertilizer. So in cold climates, the maximum nitrogen content of such solutions is usually about 28% by weight.
Urea solutions can be prepared at solids contents up to about 50% by weight. Solutions containing about 20% urea are common for lawn fertilizer applications. Such solutions also must be handled appropriately to avoid complications due to urea crystallization (salt out) at low temperatures.
Once applied to the soil, urea in such fertilizers is enzymatically converted to ammonia by urease, an enzyme produced by endogenous microorganisms in the soil. The ammonia then is hydrolyzed rapidly to ammonium ions. In the soil, some of the ammonium ions, whether from the hydrolyzed ammonia or from ammonium nitrate, are assimilated directly by plants, but most are converted to nitrate by the process of nitrification. Once in the nitrate form, the nitrogen is more readily assimilated directly by plants.
Although in widespread use, one of the problems with using urea, AN and UAN solutions as the nitrogen source is that some fraction of the nitrogen is lost after application in various ways, including volatilization as ammonia, denitrification to gaseous nitrogen and nitrate leaching. It has been estimated that the nitrogen loss for such fertilizers falls somewhere between 30% and 60%.
The fertilizer art also has developed a wide variety of liquid fertilizers based on the reaction between urea and formaldehyde. Such urea-formaldehyde resin fertilizers have been formulated and used as a way of providing a more controlled (sometimes characterized as an extended) release of the nitrogen values so that the availability of the nitrogen hopefully is tailored more closely to the time-course nitrogen requirements of the plants. In this way, it is thought that the nitrogen loss commonly associated with the quick release nitrogen fertilizers, such as urea and UAN solutions, can be reduced. However, when formulated at high solids contents to maximize the total nitrogen value, these liquid fertilizers sometimes present their own stability problems.
Notwithstanding these inherent inefficiencies and potential problems, liquid fertilizers based on such formulations continue to be an attractive way of applying nitrogen fertilizers to plants. For that reason, the fertilizer art continues to search for improved compositions and ways for providing a concentrated nitrogen fertilizer liquid that is more stable and less prone to salting-out.